how to make lines of mask/image thicker using material nodes in blender
Here's a quick and dirty way to do it, that I think you can get away with based on the fact your image is black and white, and is relatively simple. The graph looks like this:

The idea is simple - make 5 separate &quot;instances&quot; of your image, offset 4 of them slightly (one in each direction), and add (or multiply in this case) them together. For the sample image that I chose, I offset by both .002 and -.002 for each of the X and Y axes, and multiplied them together (to copy the darks) with the original image. As you can see from the image below (which represents the original image), that the lines are much thinner.

As a side note, the &quot;clamp&quot; checkboxes are optional - I just often use them as a matter of habit to keep things in range and so I know there's no wacky HDR stuff going on behind the scenes.